1. Field of the Invention
The present invention relates to an electric field enhanced combustion stabilizer that utilizes an electric field and corona discharge to improve the mixing and combustion characteristics of combustor assemblies used in combustion turbines.
2. Background Information
Combustion turbines burn hydrocarbon fuels such as methane gas mixed with large volumes of air to power a turbine. The stability of the combustion and the chemical by-products, among other parameters, are dependent on how efficiently the gas is mixed with the air and the configuration of the burner elements. One example of a combustor to heat gas for a gas turbine generator is taught by U.S. Pat. No. 5,413,879 (Domeracki et al.).
The concept of imposing an electric field with a combustion turbine combustor, to augment the mixing and configuration stabilizing elements appeared attractive based on relatively low pressure, low velocity experimental work, in particular that of Calcote, such as Calcote, H. F. & Pease, R. N., “Electrical Properties of Flames,” Industrial & Engineering Chemistry, Vol. 43, December 1951, pp. 2726–2731; Calcote, H. F. “Ion Production and Recombination in Flames,” 8th International Symposium on Combustion, LOC 55-9170, Williams & Wilkins 1962, pp. 184–199; Calcote, H. F., Berman, C. H., “Increased Methane-Air Stability Limits by a DC Electric Field,” Fossil Fuel Combustion Symposium, Houston Tex., January 1989, vol. PD 25, pp 25–31; and Berman, G. H., Gill, R. J., Calcote, H. F., and Xiong, T. Y., “Enhanced Flame Stability Using Electric Fields.” Final Report May 1991–1992, Aero-Chem Tp-511 Final Report for the Gas Research Institute, April 1993. The major focus of the last work was to screen the feasibility of applying electric fields to control combustion in practical combustors. Three combustion systems were chosen for study; two industrial burners: an IGT low NOx, 1×106 Btu/h cyclonic burner and a General Electric research gas turbine burner; and a residential commercially available GE home range burner. The major effort was on the IGT Industrial Burner. Some conclusions were that a dc electric field can improve flame stability at high excess air operation in a cyclonic combustor; that application of an electric field using a torus electrode improved flame stability in an IGT cyclonic combustor, as evidenced by reduced emissions of both CO and total hydrocarbons without affecting the already low levels of NOx; and that negligible electric power is required for stabilization in large systems. They also concluded that the major effect of the dc electric field was to prevent the increase in CO and total hydrocarbons that often occurs when a burner is operated out of its stable design range by attaining more complete combustion.
A variety of work has been done on internal combustion engines, utilizing corona discharge between spark gap electrodes; utilizing convection, and applying an electric field to segregate large fuel particles; and working with air-fuel mixtures to utilize very lean mixtures and to reduce the quantity of exhaust gases, as described in U.S. Pat. Nos.: 4,041,922; 4,124,003; 4,020,388; and 4,219,001 (Abe et al.; Abe et al.; Pratt; and Kumagai et al. respectively). Biblarz et al., in U.S. Pat. No. 4,439,980, taught electrodynamic control of fuel injection into aircraft gas turbines to allow use of fuels having high aromatic content. Most of these patents deal with air and droplets of fuel of various types.
The use of electrostatic fields to control the shape and thermophysical characteristics of flames has been established, for example, by Calcote and Pease in their 1951 article, cited previously. However, using electric fields to significantly impact high velocity turbulent flames has not been demonstrated. Prior art has assumed that improvements to flame stability require kinetic mixing of gases whether by mechanical or electrical means. The efforts at improving stability by electrical means were directed at changing the apparent burning velocity to anchor the flame to the burner. Prior art, as described in “Electrical Control of Gas Flows in Combustion Processes”, by Lawton, J., Mayo, P. J., and Weinberg, F. J., Proc. Roy, Soc. 1968, vol. A303, pp. 275–298, concluded that it is not possible to change the burning velocity by more than about 5 m/s, therefore it was thought that as the gas velocity increases above this limit, electric fields become increasingly less effective.
Johnson, in PCT International Application WO 96/01394, discusses electrode arrangements for use in a combustion chamber having a flame zone located between the electrodes where a corona discharge ionizes the air used for the combustion process. Combustion is affected in the flame zone, reducing smoke particles, hydrocarbons, carbon monoxide and nitrous components in the exhaust gas. The object of that invention was to obtain devices which provide efficient combustion in a combustion chamber with open flame combustion to reduce harmful substances in the exhaust gas, as well as to form electrical and electromagnetic, discharges and conditions which influence combustion reactions to proceed in an optimum manner to reduce emissions. The frequency of pulsed direct current was thought important for controlling the discharge process.
The field development of processes and mechanisms to increase the effectiveness of an electric field in a combustor, so that it can influence flames in the high velocity turbulent region of the combustion process, would be commercially desirable to improve operation of combustion turbines.